This invention relates to a process for the production of semiconductor grade polycrystalline silicon and more particularly to a cost efficient process for producing polycrystalline silicon using trichlorosilane and silicon tetrachloride.
In the semiconductor industry relatively large quantities of high quality monocrystalline silicon are required for the production of semiconductor devices and integrated circuits. One of the most common methods used in the growth of monocrystalline silicon is the Czochralski method in which extremely pure polycrystalline silicon is converted to monocrystalline silicon. In the conversion process, controlled amounts of dopant materials are added to the polycrystalline silicon to produce monocrystalline silicon of the desired resistivity.
The extremely pure polycrystalline silicon can be produced by the hydrogen reduction of SiHCl.sub.3 at a temperature of about 1100.degree. C. Hydrogen and SiHCl.sub.3 are introduced into a reaction chamber and polycrystalline silicon produced by the reaction is deposited in rod form on a heated filament. The reaction as commonly used is very incomplete. The exhaust gases from the reaction chamber contain large quantities of unreacted SiHCl.sub.3 and H.sub.2 as well as HCl and SiCl.sub.4. Thus only a portion of the SiHCl.sub.3 source is converted to silicon while part is unreacted and part is converted to HCl and SiCl.sub.4. The exhaust gases can be separated to remove the SiHCl.sub.3 and H.sub.2 which are returned to the reaction chamber. The SiCl.sub.4 is essentially a waste product, except that some portion of it can be used in epitaxial deposition systems and some can be converted back to SiHCl.sub.3 (Rodgers U.S. Pat. No. 3,933,985). The SiCl.sub.4 has not been used directly in the polycrystalline production process because the production of silicon from SiCl.sub.4 is too slow to be economical.
As commonly practiced, the production of polycrystalline silicon is an expensive process because of the large amounts of pure SiHCl.sub.3 and hydrogen that are used. This is aggravated by the imcompleteness of the reduction reaction. Although part of the unreacted components can be purified and reused, that portion of the SiHCl.sub.3 which is converted to SiCl.sub.4 is either wasted or must be converted back to SiHCl.sub.3 for use.
The polycrystalline silicon production process is also expensive from the standpoint of energy usage. Much energy is radiated from the conventional silicon reactor because the reaction is relatively uninsulated. The lack of insulation is necessitated by the large temperature gradient required from the silicon rod to the reactor wall. The gradient is necessary to keep the rod hot enough for the rapid deposition of dense silicon while keeping the wall cool enough to prevent deposition thereon. Deposition of silicon on the quartz walls of the reactor would lead to devitrification of the quartz and consequently to a short useful life for the quartz. In contrast, Pauls (U.S. Pat. No. 2,943,918) teaches the deposition of the polycrystalline silicon directly on the walls of the quartz reaction chamber instead of deposition on a heated filament. Recovery of the polycrystalline silicon then requires the breaking away of the quartz to free the polycrystalline silicon.
Accordingly, in view of the large amount of silicon required in the semiconductor industry, and further in view of the present emphasis on the conservation of energy, a need existed for an efficient, economical process for the production of high purity polycrystalline silicon.